SS-31

Origin and Structure of SS-31

SS-31 belongs to the Szeto-Schiller (SS) peptide family, rationally designed by Hazel H. Szeto and Peter W. Bhatt at Weill Cornell Medical College to selectively target mitochondria. The molecule consists of just four amino acid residues: D-Arg-Dmt-Lys-Phe-NH₂, where Dmt denotes 2',6'-dimethyltyrosine. The alternating aromatic-cationic motif is the key structural element enabling over 1000-fold accumulation of the peptide in mitochondria.

The molecular weight of SS-31 is only ~639.8 Da, making it one of the smallest mitochondria-targeted peptides. D-arginine at position 1 provides resistance to protease degradation. Dimethyltyrosine at position 2 is responsible for cardiolipin interaction. C-terminal amidation (-NH₂) protects the peptide chain from carboxypeptidases.

Mechanism of Action

SS-31 selectively accumulates in the inner mitochondrial membrane (IMM), where it binds to cardiolipin, a phospholipid found almost exclusively in this structure. Cardiolipin stabilizes the organization of electron transport chain (ETC) supercomplexes and is essential for proper cytochrome c function.

In in vitro cellular models, binding of SS-31 to cardiolipin was shown to optimize electron transfer within the ETC, limiting electron leak and reactive oxygen species (ROS) production. Importantly, SS-31 does not act as a classical uncoupling protonophore: it does not dissipate the mitochondrial membrane potential (ΔΨm), but only improves electron flow efficiency.

This mechanism distinguishes SS-31 from "scavenger" type mitochondrial antioxidants (e.g., MitoQ), which neutralize already-formed ROS. SS-31 acts at an earlier stage, limiting the very process of excessive free radical generation at its source.

Research Overview

Discovery and design of SS peptides

The Szeto-Schiller peptide family was described by Hazel H. Szeto in a review in AAPS Journal (2006, 8(3):E521-E531). The author presented the rational basis for designing peptides with alternating aromatic-cationic motifs that selectively accumulate in mitochondria ^[1].

Zhao, Zhao, Wu, et al. in Journal of Biological Chemistry (2004, 279(33):34682-34690) demonstrated that SS peptides penetrate cell membranes and accumulate in the inner mitochondrial membrane, inhibiting mitochondrial swelling and limiting cell death induced by oxidative stress ^[2].

Cardiolipin binding

Birk, Liu, Soong, et al. published in J Am Soc Nephrol (2013, 24(8):1250-1261) a key paper confirming direct SS-31 binding to cardiolipin. In a rat kidney ischemia-reperfusion model, they demonstrated restoration of mitochondrial bioenergetic efficiency through restabilization of ETC supercomplexes ^[3].

Animal model studies

SS-31 has been studied in mouse and rat models of cardiac ischemia-reperfusion and kidney injury, with observed reduction in mitochondrial dysfunction. In aging mouse models, improved mitochondrial bioenergetic parameters were documented in tissues of aged animals ^[2,3].

Structural Features

SS-31 is a tetrapeptide with a mass of ~640 Da featuring several key modifications:

• D-Arg (position 1) — D-isomer of arginine, increasing protease resistance and providing cationic charge (+)
• Dmt (position 2) — 2',6'-dimethyltyrosine, a modified tyrosine with greater lipophilicity facilitating mitochondrial penetration
• C-terminal amidation — Phe-NH₂ increases stability and eliminates the negative charge of the carboxyl group
• Net charge +3 — D-Arg (+) and Lys (+) give the peptide a cationic charge favoring mitochondrial accumulation

Intended Use

This product is a chemical raw material intended exclusively for in vitro research, scientific, and analytical applications. It is not intended for human or veterinary use, including for diagnostic, therapeutic, prophylactic, or nutritional purposes.

Handling

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Regulatory Classification

This product is not classified as a hazardous substance under CLP Regulation (EC) No 1272/2008. It is offered as a chemical reagent within the framework of applicable European Union chemical substances regulations (REACH, CLP). It does not constitute a medicinal product, dietary supplement, medical device, or cosmetic within the meaning of applicable laws.

Disposal

Unused material should be disposed of in accordance with local regulations on chemical waste. Under laboratory conditions: neutralize and dispose with laboratory waste streams. Do not discharge into sewage systems.

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